The electrical utility companies have shown considerable interest in obtaining detailed information concerning the nature and amount of electrical usage by their customers. As utilities attempt to reduce costs, and deliver electrical power to customers in the most efficient manner possible, detailed and reliable data concerning the needs and usage of power by the customers becomes very important. Additionally, unusual variations in the amount or nature of power usage could indicate a developing problem in the delivery of power, allowing the utility to intervene with appropriate maintenance procedures prior to the onset of power shortages. Major power consuming customers also are interested in having more detailed and reliable information concerning their own electrical usage in an attempt to increase their efficiency of power usage and to hold down costs.
The basic electrical quantities typically measured directly are the current and voltage. From these fundamental measurements other quantities, such as power, are calculated. Therefore, the accuracy and reliability of the fundamental measurements of voltage and current set inherent limits on the accuracy of power determinations and other electrical quantities calculated from current and voltage. The subject of the present invention is to present a method and apparatus for increasing the accuracy of current measurements under various conditions of ac, dc and combined ac and dc current flows.
A common device for the measurement of ac currents is the "current transformer". The current transformer ("ct") consists essentially of a substantially toroidal shaped core of magnetic material around which are wrapped a number of turns of insulated electrical conductor, the "windings". The conductor carrying the current to be measured passes axially through the ct, resulting in the current to be measured being surrounded by the toroidal structure of the ct. The voltage induced in the windings is measured to ascertain the current passing axially through the ct.
It is a fairly sophisticated problem to design the ct to produce very accurate current measurements for the anticipated ac current magnitudes and frequency ranges. For example, the core material, the core geometry, and number and size of the windings are all considerations in producing accurate current measurements. However, by careful attention to all aspects of the ct design, it is possible to achieve accuracies of current measurements in the range of better than 0.01% for a particular frequency range (typically 50 to 60 Hz as typically necessary for measurement of current in power distribution systems).
However, the accuracy of typical current transformers is often seriously degraded when significant dc components are present in the current passing through the ct. The presence of dc current produces a substantially constant magnetic field superimposed on the time-varying magnetic field generated by the passage of ac current axially through the ct core. This dc component of magnetic field tends to cause saturation in the magnetic core of the ct, leading to very serious errors introduced into the current readings produced by the ct. Typically, when the dc current comprises approximately 5% or more of the total root-mean-square ("rms") current to be measured, serious errors in ct current measurements may be anticipated. The error of ct ,current measurements as a function of dc current component is generally a complex function of the design of the ct as well as the nature of the ac current present. However, for most cases of practical interest, the error of ct current measurements is a steeply rising function as increasing proportions of dc current is present.
Another common method for current measurements is based upon the "Hall effect". Basic electromagnetic theory teaches that moving charges will experience a force when placed in a magnetic field. In particular, charges carrying current will experience forces perpendicular to the current flow when placed into a magnetic field whenever the applied magnetic field has a component perpendicular to the current flow. These forces experienced by the charges carrying the current are perpendicular both to the current flow and to the (perpendicular component) of the applied magnetic field. The well-known "right-hand rule" of electromagnetic theory gives the direction of the force on the charges in terms of the direction of the magnetic field and the direction of motion of the charges (i.e. the direction of current flow). Forces tending to displace the current-carrying charges in a direction perpendicular to the current flow will generate a voltage perpendicular to the current. The Hall effect typically utilizes a current flowing through a thin conductor with a magnetic field applied perpendicular to the flat face of the conductor. A voltage will be produced perpendicular to both the current flow and applied magnetic field. The size of this Hall effect voltage is a direct measure of the applied magnetic field.
Hall effect current sensors are commercially available devices. Essentially, they typically employ a substantially toroidal core of magnetic material having a relatively thin gap radially through the torus; essentially similar to a toroidal-shaped magnetic with facing pole pieces. A thin piece of material carrying current is placed in this gap and voltage measurement devices measure the Hall effect voltage generated across this material. The conductor carrying the current to be measured passes axially through the toroidal core of the Hall effect sensor. The magnetic field generated by the current to be measured changes the magnetic field (and hence the voltage) generated by the Hall effect across the current-carrying conductor.
Such Hall effect sensors can measure dc as well as ac currents. However, they suffer from the disadvantage that the current measurements are not as accurate as desired; typically 5-7% error in current values. Thus, the present invention is a combination of ct and Hall effect sensors combined in such a manner as to use the ct sensor for the range of ac currents in which it is known to be highly accurate while using the Hall effect sensor when the ct sensor is known to give unreliable results. The method and apparatus for switching from Hall effect to ct sensors so as to achieve maximum practical current measurement accuracy under ac, dc as well as combined ac-dc currents, is the subject of the present invention.